Previous research suggests that personality and social behavior in individuals with Down Syndrome (DS) differs from that in indiviuals with other forms of mental retardation. Although these studies have concluded that greater adjustment and social responsiveness are characteristic of individuals with DS, serious methodological problems make this conclusion premature. The proposed research is an investigation of personality and social behavior in DS that avoids the methodological problems of previous research. Several aspects of the proposed research are significant: Personality and social behavior will be assessed in naturalistic, semi-structured interaction sessions, naive raters being used to generate ratings of personality and behavior from videotapes of these sessions. The behavior of DS children will be compared with that of matched children with other forms of mental retardation and non-retarded children of matched mental age. These (and other) features of the proposed research ensure that personality and social behavior in DS will be examined in a significantly more conclusive manner than has been true in previous research. If personality and social behavior in individuals with DS is found to be reliably different from that in individuals with other forms of mental retardation and non-retarded individuals, a foundation will be provided for research in two important areas: the interaction of genetic and environmental influences in the development of personality and social behavior, and the design of educational and social treatment programs that take into consideration the special characteristics of the individual with Down Syndrome.